I may not be the right one to try to answer this since I never use a phone for access, I seldom change my forum "skin", and the options I see might not be the same as yours. Still...

When I look at the Quick Style Chooser at the bottom left of this page, the options I see include "Psych Central" (showing), "Desert Sand", "Spring", "Tulip", "Teal / Black", and "Default Style". Nothing about mobile.

When I go to User Control Panel > Edit Options > Miscellaneous Options > Forum Skin, I see a longer list that does include "Mobile".

What do you see?